Precursor solution was prepared using ammonium vanadate (1.17 g) and ammonium dihydrogen phosphate (1.15 g). The salts were mixed in 100 ml deionized water, bringing the concentration of the solution to 0.1 M. 1.5 g of sucrose was added to the solution as an organic component. The addition of the organic component was to produce a self-sustaining flame during the FSP process. The solution was stirred at 600 RPM and maintained at 60°C using a heated magnetic stirrer until a homogenous mixture was obtained. The solution was aged for a day and then fed into the FSP system (Tethis NPS 10). For the FSP process, the flame was generated using methane and oxygen, 1.5 slm methane and 3.0 slm oxygen were used, 5 slm oxygen was used as dispersion gas and the solution feed rate was 5 ml. The particles were collected on a glass fiber filter. The powders were calcined at 500°C for 8 h.

Precursor solution was prepared with the same amount of ammonium vanadate and ammonium dihydrogen phosphate. The salts were mixed in 80 ml deionized water. The stirring and heating of the precursor was performed the same as mentioned earlier. A total of 20 ml of DMF, which was used as the organic compound of this solution, was added to make it 0.1 M, which is the same concentration of the other precursor made with sucrose; then, it was fed into the FSP system. The FSP process was done with the same parameters as mentioned in the previous part.

The powders were characterized using Scanning Electron Microscopy (SEM, LEO 1550), Transmission Electron Microscopy (TEM, JEOL 1400) and selected area electron diffraction was carried out on both as synthesized and calcined samples. Thermogravimetric analysis (TGA, Perkin Elmer Diamond) was conducted on the as-synthesized samples. The particles, both as-synthesized and calcined, were also characterized using XRD (Rigaku Miniflex II) and FTIR (Thermo Scientific Nicolet 6700).

Figure 1 shows the XRD characterization of FSP-synthesized powders. The as-synthesized particles were relatively amorphous showing significant broad peaks; some extent of crystallinity was obtained. The powders synthesized using a sucrose-based solution, matched with JCPDS 36-0054 for Ammonium Vanadyl Hydrogen phosphate hydrate (NH₄VOVO₂(HPO₄)₂.1.5H₂O) and are denoted by the letter A in Figure 1a. Figure 1a also shows the pattern for powders synthesized using a DMF based solution. The pattern showed a mixed system and matched with JCPDS 36-0054 for Ammonium Vanadyl Hydrogen phosphate hydrate(denoted by A) and JCPDS 34-1247 for tetragonal VOPO₄ system(L → R denoted by α, 200, and 301 planes). The tetragonal VOPO₄ system was identified to be α_II-VOPO₄ based on the lattice parameters (Ling et al., 2014).

Figure 1b shows the XRD characterization of calcined FSP synthesized powders. Both the powders resulted in mixed phases. In the case of calcined powders synthesized with sucrose-based solutions, the peaks from the XRD pattern were found to be orthorhombic β-VOPO₄ (JCPDS 71-0859) and tetragonal α_II-VOPO₄ (JCPDS 34-1247). The peaks for β-VOPO₄ are denoted by β(L → R: 011, 002, 201, 221, 031, 400, and 040 planes) and the peaks for α_II-VOPO₄ are denoted by α(L → R: 101, 200, 220, and 132 planes). For calcined powders synthesized with DMF-based solutions, the peaks from XRD pattern were found to be tetragonal δ-VOPO₄ (JCPDS 47-0951) and tetragonal α_II-VOPO₄ (JCPDS 34-1247). The peaks for δ-VOPO₄ are denoted by δ(L → R: 002, 111, 012, 022, 031, 214, and 106 planes) and the peaks for peaks for α_II-VOPO₄ are denoted by α(L → R: 101, 111, 200, 220, 112, 301, 311, and 420 planes).

The relative phase content was calculated using peak intensity ratios for all the samples. Quantitative analysis for crystallite size determination was conducted using Scherrer's formula. Table 1 lists the phase content and their crystallite sizes.

TGA-DTA was conducted on as-synthesized samples in the range of RT-550°C under oxygen atmosphere. Figure 2 shows the plot for (a) powders synthesized using sucrose-based solution and (b) powders synthesized using DMF-based solution.

Powders synthesized using sucrose-based solution show a decrease in weight with increase in temperature. The weight loss can be associated with the release of ammonia and water by decomposition of NH₄VOVO₂ (HPO₄)₂.1.5H₂O and formation of VOPO₄ structures. The DTA signals show two exothermic changes at 260 and 540°C; these changes can be associated with formation of β-VOPO₄ and α_II-VOPO_4.

Powders synthesized using DMF-based solution also show a decrease in weight with increase in temperature. Similar to the sucrose-based solutions, the weight loss can be associated with the release of ammonia and water by decomposition of NH₄VOVO₂(HPO₄)₂.1.5H₂O and formation of VOPO₄ structures. The weight drop stops at 420°C and a slight spike in weight is noticed in the range of 420–550°C. The DTA signals show an exothermic change at 420°C. Since the as-synthesized particles were a mixture of vanadyl hydrogen phosphate hydrate & α_II-VOPO₄, which resulted in formation α_II-VOPO₄ and δ-VOPO₄, the exothermic reaction at 420°C can be associated with the formation of δ-VOPO₄.

FTIR was conducted on all the samples in the range of 500–4,000 cm⁻¹. The signal received below 500 was erroneous due to significant noise; it was therefore omitted from the results. Figure 3 shows the FTIR spectra for (a) powders obtained from sucrose-based solution (as-synthesized and calcined) and (b) powders obtained from DMF-based solution (as-synthesized and calcined).

For the as-synthesized particles from the sucrose-based solution, the peaks were obtained at 569, 1,040, 1,424, 1,654, and a broad peak in 3,200 cm⁻¹. The peak at 1,424 cm⁻¹ and the broad peak at 3,200 cm⁻¹ correspond to the N-H bending vibrations (Socrates, 2004). The peak at 1,654 cm⁻¹ indicated presence of water. The strong peak at 1,040 cm⁻¹ belongs to P-O (Jiao et al., 1998) vibrations and the peak at 569 cm⁻¹ can be associated with V-O-V rotational vibrations (Xue et al., 2010). Upon calcination, the FTIR spectra for the calcined powders revealed peaks at 559, 673, 903, 949, shoulder at 1,080, 1,614, and a broad peak at 3,344 cm⁻¹. The broad peak at 3,344 cm⁻¹ and the peak at 1,614 cm⁻¹ correspond to N-H bending vibrations (Socrates, 2004) and signify the presence of undecomposed ammonia. The V-O-V rotational vibrations observed at 559 cm⁻¹ were observed at 569 cm⁻¹ in the calcined sample. Similarly, the peak for P-O vibrations observed at 1,040 cm⁻¹ was observed at 1,080 cm⁻¹. New peaks were observed at 673, 903, and 949 cm⁻¹. The peak at 673 can be associated with metaphosphate (Socrates, 2004). The peaks in 903 and 949 cm⁻¹ region belong to the V = O vibrations (Anumula et al., 2013). The different peak position signifies a shift in the V = O bonds, the shift can be associated with the presence of two phases.

For the as-synthesized particles from the DMF-based solution, the peaks were obtained at 566, 676, 879, 941, 1,064, 1,426, and 1,644 with a broad peak at 3,200 cm⁻¹. The peak at 1,426 cm⁻¹ and the broad peak at 3,200 cm⁻¹ correspond to N-H bending vibrations as was the as-synthesized particles from sucrose solution. Similarly, the peak at 1,644 cm⁻¹ indicated inclusion of water. The peak at 1,040 cm⁻¹ belongs to P-O vibrations, the peak at 941 belongs to V = O vibrations and the peak at 566 cm⁻¹ can be associated with V-O-V rotational vibrations. The peak observed at 873 cm⁻¹ corresponds to orthovanadates (Socrates, 2004). Upon calcination, the FTIR spectra for the calcined powders revealed peaks at 560, 674, 939, shoulder at 1,020, 1,428, and 1,612 with a broad peak at 3,254 cm⁻1. The broad peak at 3,254 cm⁻¹ and the peak at 1,612 cm⁻¹ correspond to N-H bending vibrations (Socrates, 2004) and signify slight presence of undecomposed ammonia. The V-O-V rotational vibrations observed at 566 cm⁻¹ were observed at 560 cm⁻¹ in the calcined sample. Similarly, the peak for P-O vibrations observed at 1,040 cm⁻¹ was observed at 1,020 cm⁻¹, the peak for V = O vibrations observed at 941 cm⁻¹ shifted to 939 cm⁻¹, the metaphosphate peak shifted from 676 cm⁻¹ to 674 cm⁻¹. A slight shift in peak positions and absence of new peaks indicate that no major change occurred in the structure upon heat treatment.

The as-synthesized particles and heat-treated samples were characterized by SEM and the particle morphology was analyzed. Figure 4 shows the SEM images for as-synthesized particles and Figure 5 compares the SEM images of the heat-treated samples.

The TEM imaging of calcined particles from sucrose-based solution showed thin spherical disk-like particles (Figures 7a,b). The particle sizes for these disks were in the range of 40–300 nm. The SAED pattern on the disks resulted in rings that matched with JCPDS 34-1247 α_II-VOPO₄ (Figure 7c; 200 and 112 planes). No rings were found representing β-VOPO₄; however, the imaging showed growth of lathes on the edges of the disks which might be associated with β-VOPO₄. In case of calcined particles from DMF-based solution, the morphology was in the form of rectangular nanosheets. The size of these sheets was in the range of 50–500 nm in length. The particle sizes for these disks were in the range of 40–300 nm. SAED pattern on the disks resulted in rings that matched with JCPDS 34-1247 α_II-VOPO₄ (Figures 6, 7f; 200 and 112 planes).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.